This invention relates to a magnetoinductive flowmeter with a ceramic measuring tube as a conduit for a medium whose flow rate is to be measured, and with a measuring electrode which measuring electrode, featuring a cermet body, is positioned in the wall of the measuring tube, its side facing the medium conductively or capacitively connected to the medium while its side facing away from the medium connects to a measuring line. The invention further relates to a method for producing a measuring tube for a magnetoinductive flowmeter.
Magnetoinductive flowmeters of the type referred to above have been well known for some time and are used in numerous different areas of application. The basic concept of a magnetoinductive flowmeter for flowing substances goes all the way back to Faraday who in 1832 introduced the idea of employing the electrodynamic induction principle for flow-rate measurements.
Faraday's law of induction postulates that when a flowing medium that contains charge carriers travels through a magnetic field, an electric field intensity is generated perpendicular to the direction of flow and perpendicular to the magnetic field. A magnetoinductive flowmeter applies Faraday's law in that a magnet, typically consisting of two magnetic poles, each with a field coil, generates a magnetic field perpendicular to the direction of flow in the measuring tube. Within this magnetic field, each volume element of the medium flowing through the magnetic field and containing a certain number of charge carriers contributes the field intensity generated in that volume element to a voltage potential that can be collected via the measuring electrodes.
The measuring electrodes are usually designed for conductive or capacitive inductive coupling to the flowing medium. On the side facing away from the medium they connect to a measuring line through which the collected electric potential can be fed to an evaluation device.
The use of cermet bodies for the measuring electrodes, which for the purpose of this description covers all electrodes that can be employed with the measuring tube of a magnetoinductive flowmeter, reference electrodes included, is common practice. The cermet body consists, for instance, of platinum as the metal and aluminum oxide as the ceramic component, with the ratio between the metal and the ceramic components as well as their commixture and the microstructure obtained thereby being so selected as to ultimately arrive at a conductive cermet body for the measuring electrode.
To date, cermet bodies for the measuring electrodes of a magnetoinductive flowmeter have been produced primarily by extruding a metal/ceramic mixture. Cutting the resulting extrusions into essentially pin-shaped bodies produces measuring electrodes that can be installed in the ceramic material of the measuring tube, for instance by press-fitting. This approach, however, has a number of drawbacks in that the process of cutting the cermet extrusion can result in breakage, and thus in the destruction of the measuring electrodes, involving considerable consumption of material, especially of the metal such as the expensive platinum.